Field Stop Diameter Measurement

Can anyone tell me how to measure the field stop diameter of an eyepiece? A lot of manufacturers do not list this measurement in their specs. Thank you for your time, Rick Miseroy

If the field stop is exposed in the barrel, you can measure it directly. If it is hard to get to with the calipers, you can cut off the cotton ends from a Qtip and insert the stick or something similar with a pair of needle nose pliers (careful!) and then mark the stick with a pen, then remove and measure on the stick.

If the field stop is behind a lens element, the field stop diameter is irrelevant without knowing how to account for the lens element in front of it. Your only option is to measure the true field of view and "back into" the field stop diameter:

field stop diameter ~ true_field_degrees * telescope_focal_length / 57.3

Finally, if you only want something quick and dirty, you can assume that TFoV = AFoV / Magnification, which yields the crude approximation:

field stop diameter ~ apparent_field_degrees * eyepiece_focal_length / 57.3

I often use this last approximation in spreadsheets where I prefer to use field stop data when I have it but am willing to use the crude approximation when I don't.
--
Mike

You might also check that old Dietzgen or Alvin drafting set for a pair of dividers. The screw adjust type work excellent. Use extreme caution when near lenses! Follow the formulas that Mike provided. You need measurements to fractions of a mm.

edz